1. Field of the Invention
The present invention relates to circuitry for the excitation of filters in electrical protection devices such as line protectors, generator protectors, motor protectors, and the like.
2. Prior Art
In excitation circuits where the peak or mean value of two or three-phase currents are measured by circuits comprising current transformers, rectifiers and smoothing devices, there are normally required additional transformers for the excitation of filters, such as for example positive or negative sequence filters. The excitation voltage of such filters is required to have exactly the same shaped characteristic as that of the primary side of the current transformers and such excitation voltage must not be influenced by the loads in the current measurement circuits. Also the filter excitation must not affect the measurement of the phase currents.
According to a known method of filter excitation, which avoids additional current transformers, an excitation voltage is obtained which is a direct image of the primary current of the current transformer by sensing the current through a rectifier bridge using a low-ohmic resistor and a differential amplifier. However, that excitation voltage is related to the signal zero and the positive polarity of the rectifier signal alternatively every second half cycle, which complicates the method of current measurement, as the sinusoidal input signal is superimposed on a square wave, the amplitude of which varies with the current.